Many oil reservoirs have been discovered which contain vast quantities of oil, but little or no oil has been recovered from many of them because the oil present in the reservoir is so viscous that it is essentially immobile at reservoir conditions, and little or no petroleum flow will occur into a well drilled into the formation even if a natural or artificially induced pressure differential exists between the formation and the well. Some form of supplemental oil recovery must be applied to these formations which decreases the viscosity of the oil sufficiently so that it will flow or can be dispersed through the formation to a production well and therethrough to the surface of the earth. Thermal recovery techniques are quite suitable for viscous oil formations, and steam flooding is the most successful thermal oil recovery technique yet employed commercially.
Steam may be utilized for thermal stimulation for viscous oil production by means of a steam drive or steam throughput process, in which steam is injected into the the formation through an injection well, to heat the formation and, in so doing, to reduce the viscosity of the oil and, possibly also, to induce a degree of cracking, resulting in a further reduction in viscosity. Processes of this type can be generally classified as basically of the two-well or one-well type. In the two-well or steam-drive type, the steam is injected through an injection well, and the injected steam serves to drive the oil towards a separate production well, which is located at some horizontal distance (offset) from the injection well. In the one-well or "huff-and-puff" type operation, a single well is used for both injection and production. The steam is first injected to reduce the viscosity of the oil and to pressurize the formation; after a certain amount of time, steam injection is terminated and the well is turned over to production. A soak period may be allowed to permit the heat to permeate the reservoir to a greater extent before production is initiated in either type of operation. Whether the process is classified as of the one-well or two-well types, the well arrangement can, of course, be repeated to cover the field in the manner desired. For example, the two-well arrangement may be repeated in regular patterns; such as, the inverted five spot or inverted seven spot patterns, as described in U.S. Pat. No. 3,927,716. The present method relates basically to the two-well type operation, using an injection well or wells and a separate production well or wells at an offset from the injection well, as described in U.S. Pat. Nos. 3,500,915; 4,431,056; and 4,456,066.